1. Field of the invention.
This invention relates to improvements in apparatus for generating pulses for heart stimulation, and more particularly to improvements in apparatus which generates pulses for heart stimulation in the absence of naturally occurring heart pulses.
2. Description of the prior art.
The living heart effects its pumping action with many complex muscle contractions and relaxations. Typically, such contractions and relaxations are accompanied by electro-chemical potential pulses which can be analyzed and associated with particular heart muscle actions. For example, the contraction of the ventricle heart muscle produces a positive pulse, ordinarily of larger magnitude and time duration than the other heart-produced pulses. Thus, the ventricle muscle contraction presents a unique frequency spectrum which can be detected and distinguished from the other heart-produced pulses.
As a result of disease, injury, or other cause, occasionally a heart fails to function properly; for instance, one or more muscles may fail entirely to properly contract and relax, or may intermittently fail to properly function. In such cases, artificially produced electrical stimulation pulses may be effective in restoring the proper function to the failing muscle.
Many devices have been proposed for generating such pulses to stimulate a naturally occurring heartbeat. The devices which operate to produce stimulation pulses when the heart fails but which otherwise remain quiescent or inactive, such as the device disclosed in U.S. Pat. No. 3,345,990, are commonly referred to as "demand" heart pacers, and generally include one or more electrical conduction leads physically implantable in the heart to detect the presence or absence of the naturally generated electrochemical potential associated with a particular muscular function and apply electrical stimulation pulses to simulate the natural pulses in their absence. Demand pacers such as disclosed in U.S. Pat. No. 3,253,595 have been advanced which are entirely implantable within the body and include solid state circuitry potted in a unit having a surrounding conductive portion to establish electrical contact with the body at a remote point, such as in the abdomen. A single lead is provided for connection to the heart at an appropriate location.
Of the demand heart pacers proposed heretofore, many employ circuits which are continuously conducting, such as monostable multivibrators which switch from a first conducting state, for instance through one transistor, to a second conducting state, through a different transistor. Thus, the multivibrator may draw current continuously from the voltage source, which can shorten its useful lifetime and require frequent replacement and attention.
Additionally, circuits of the prior art commonly employ relatively complicated circuitry to distinguish the various heart pulses, heart pacers commonly being triggered upon the presence or absence of the so-called "QRS" complex heart pulses.
Finally, demand heart pacers have been proposed which employ multivibrator apparatuses to generate periodic heart stimulation pulses at the multivibrator frequency, but upon detection of the triggering naturally occurring heart pulse, such as the QRS complex above mentioned, the output from the multivibrator is switched to a substiture load, the multivibrator being allowed to continue its periodic conduction as if it were still supplying heart stimulation pulses to the heart itself. This also results in current from the supply to flow through the transistors or current regulating devices of the multivibrator and is another source of unnecessary power drain from the supply voltage source.
Becoming of recently increasing interest, nuclear batteries are being employed to supply power to heart pacer circuits. Nuclear batteries, however, present problems in the circuit design not ordinarily encountered in the use of ordinary chemical batteries, the voltage supply, for example, being more dependent upon the impedance of the circuit to which the voltage is applied, and, additionally, being of relatively lower magnitude than a conventional chemical type battery of otherwise same relative characteristics. The use of a nuclear battery, therefore, requires particular circuitry design to obviate these problems.